This site could really do with a face palm emoticon.
how on earth are they offended that I think their God is imaginary?
It's not about your opinion. Most people are aware atheists believe there is no god. As I have said, the manner in which it is said, is to mock and that offends.
No, they may not care that you do not agree with circumcision. But they may be offended to be called 'child abusers' which I have also seen. As that is obviously said to offend.
would you risk offending me by maybe having a word that fluffy talking bunnies aren't real and mutilating babies in the name of them is futile, at best?
If it isn't illegal. Do what you will.
If you feel strongly enough about what the fluffy bunnies are saying to you that you would slice the tip off your newborn. There is hardly anything I can say to make you change your mind.
If I did say something, I would engage you in conversation. Not insult you.